The present invention relates generally to heat transfer elements, and more particularly, to a liquid cooled heat exchanger having a plurality of heat transfer fins.
Continued growth in the power density of electronic power devices correspondingly gives rise to increasing heat dissipation requirements as well. Current power electronic devices have heat dissipation requirements much greater than those of just a few years ago. Indeed, the thermal environment of current power electronics is such that overheating has become a major concern.
It is known to use liquid convection heat transfer to cool power electronic devices using synthetic polyalphaolefin (PAO) oil or other mediums as a coolant. The PAO coolant is directed through conduits placed in thermal communication with one side of a dielectric substrate having high thermal conductivity. The electronic components are to the other side of the dielectric substrate and are thus cooled by the heat flux directed through the dielectric substrate and into the PAO coolant.
While this method of heat removal is somewhat effective, a need for improvement exists. More specifically, high thermal stresses can occur from soldering a typically high coefficient of thermal expansion (CTE) heat exchanger to a substrate as well as subsequent device operation which can cause undesirable deformation of the typically low CTE dielectric sheet. And, while conducting the PAO coolant through the conduits provides for some heat removal, as component densities and power output increase, a more efficient and effective cooling mechanism becomes necessary.
It is well known in the cooling art to provide heat transfer fins to enhance the efficiency of heat transfer. The cooling fins found on air-cooled power equipment such as lawn mowers would be a good example of this technique. The cooling fins serve to increase the surface area upon which heat transfer can take place, dramatically improving the overall cooling effectiveness. While simply adding cooling fins to the dielectric sheet would seem to be a convenient solution, in practice, this approach is problematic. Good thermal conductors, (i.e. fin materials such as copper or aluminum) often have high CTEs. Good electronics substrates have much lower CTEs (xcx9c⅓ copper or aluminum) so as to match the CTE of the electronics material, silicon. While soldering a fin material to the electronic substrate, can increase heat transfer, it can also introduce a disadvantageous increase of thermal stress within the device due to CTE mismatch caused by initial fabrication and also by subsequent, inefficient heat transfer during operation. These thermal stresses can result in deformation of the dielectric sheet. In a worst case scenario, an excessive deformation could cause the disconnection of some fins from the substrate whereby the heat transfer performance of the device degrades significantly. Micro cracking of the substrate may also occur whereby the dielectric quality degrades and the device is destroyed via an electrical short circuit. While the thermal stress may be reduced by attaching the fin structure by press contact in lieu of soldering, the heat transfer effectiveness would be dramatically reduced by the increased thermal contact resistance.
A need exists therefore for an improved heat transfer device capable of high heat flux operation for cooling high heat flux power electronic devices. Such a heat transfer device would be finned and soldered directly to the substrate for providing highly efficient, reliable high heat flux operation, throughout the range of expected operating conditions yet would not introduce undesirable thermal stresses into the electronic power device during operation.
Accordingly, it is a primary object of the present invention to provide a finned heat exchanger overcoming the limitations and disadvantages of the prior art.
Another object of the present invention is to provide a finned heat exchanger to provide high heat flux operation.
Yet another object of the present invention is to provide a finned heat exchanger incorporating a unitary fin array that is simple and inexpensive to produce yet can be manufactured from a single sheet of material.
Still another object of the present invention is to provide a finned heat exchanger incorporating a unitary fin array and a closed coolant channel for directing a flow of liquid coolant.
Additional objects, advantages and other novel features of the invention will be set forth, in part, in the description that follows and will, in part, become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention as described herein, a finned heat exchanger includes a plurality of spaced, adjacent fin banks for providing effective heat transfer.
The preferred embodiment of the finned heat exchanger selected to illustrate the invention includes a unitary fin array. The unitary fin array includes a multiplicity of adjacent fin banks each having a plurality of raised fins for heat transfer. Each of the fin banks are oriented along a transverse direction and are spaced apart from each other in the longitudinal direction.
According to an important aspect of the present invention, the multiplicity of fin banks are each retained within the overall unitary fin array by looped expansion turns at the ends of each of the fin banks. The looped expansion turns provide the dual advantages of retaining the layout and orientation of the fin banks and serving to minimize the buildup of any undesirable thermal stresses within both the fin array and the device that the fin array is attached to. The fin array can be readily manufactured to any size and include any number of fins to accommodate the maximum number of heat transfer applications.
The unitary fin array is attached to a thermally conductive base. The component to be cooled. is attached to the other side of the base. In the preferred embodiment, the base is a dielectric substrate to provide maximum heat transfer while electrically insulating the electrical components from each other. A cap is attached to the base to create a closed coolant flow channel. In the preferred embodiment, the coolant is PAO, a dielectric, synthetic oil coolant ideal for use in electronic devices, however, the finned heat exchanger of the present is not considered limited to any particular coolant.